riscv: entry: gate shadow call stack reload on sstatus.SPP, not tp

In the Linux kernel, the following vulnerability has been resolved:

riscv: entry: gate shadow call stack reload on sstatus.SPP, not tp

Under CONFIG_SHADOW_CALL_STACK, handle_exception() reloads the kernel
shadow call stack pointer (gp) only when the trap came from userspace.
It made that decision with scs_load_current_if_task_changed, which
compares the trap-time tp (read back from sscratch into s5) against the
current tp:

beq \prev, tp, _skip_scs
scs_load_current
_skip_scs:

tp is a user-writable register. A user task that sets tp to the
linear-map address of its own task_struct (&current) before trapping
makes the comparison succeed (can achieve by brute force with KASLR
enable under our test), so the kernel skips scs_load_current and
keeps running with the gp that userspace left in place. Under
CONFIG_SHADOW_CALL_STACK gp is the shadow call stack pointer and
load_global_pointer is a no-op, so the compiler-emitted
"sd ra, -8(gp)" / "ld ra, -8(gp)" in every non-leaf function then write
to, and return through, an attacker-controlled address -- an
unprivileged kernel arbitrary-write and control-flow-hijack primitive.
The poisoned gp is saved to task_struct.scs_sp on return, so the reload
is skipped on every subsequent entry as well.

Decide "came from userspace" from the hardware sstatus.SPP bit (already
in s1 at this point) instead of from tp, mirroring the check that
ret_from_exception already uses a few instructions later. SPP is
written by hardware on trap entry and cannot be forged from U-mode.
Legitimate behaviour is unchanged: user entries (SPP=0) reload the task
shadow call stack, nested kernel entries (SPP=1) skip it.